[Android WebView] Fix invalidate corner case

We can never assume that calling View invalidate will lead to a draw,
since view can be invisible, off screen, clearView, etc so draw will
be optimized out. Therefore must always ensure that the fallback tick
matches what the compositor expects, ie if compositor expects a
BeginFrame, then we must always have a pending tick.

One case where this does not hold true is if a DrawGL clip does not
cover the entire visible area, we call invalidate, set block_invalides
to true, but does not schedule a fallback tick. Until the next draw
comes, webview remains in a live lock situation that no more fallback or
invalidates will be called.

Fix by not setting block_invalidates in this case so subsequent fallback
ticks can still be scheduled.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/25764003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226676 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226676